Which is the most economical kind of small vessel?

I think that depends on the length of the mast, the sea state and the running depth. Besides it is hard to argue with success.

http://www.ise.bc.ca/dolphin.html and http://www.ise.bc.ca/dorado-old.html

Regards;
Mike Schooley

 

Semi-submersible

This is an interesting thread for me. I was introduced to this subject by a photograph in a British newspaper of a semi-submersible that a man in Bristol had built. The vessel was made of steel, as I recall, and was actually in the water in a Bristol dock. A relative in the area tells me that the boat was later put on a trailer and used as an advetisement for a pub! I have tried to find out more about it but have not been able to. Anyone know anything about it? Did it work?

My interest is in making a survivable craft that can routinely cross the atlantic in any weather. Surface craft of almost any size are at risk. (the excerpt from the skipper's log on the Cutty Sark in London has a phrase that goes someting like this: "In all my 30 years of sailing I never saw a wave like that". The wave nearly sank the vessel. Also, the QE2's skipper tells of the day they were hit by a 90 foot wave). I was thinking of a boat that would let the waves break over the top and keep the vertical G's down.

My concept would be an almost cylindrical hull about 6 to 7 feet in diameter, made of steel with a stabilizing weight (keel?) suspended a few feet below. The craft would be self-righting, of course, but should not roll much even in beam seas. A snorkel (with a good active radar!) would stick up to let the small diesel breathe. If I could get (say) 10 miles per gallon of fuel, then a 2500 mile run across the Atlantic would use 250 gallons which, at present diesel prices, would only cost around $500. But going slowly with a nice big prop might yield 20mpg. I have no real idea how low the fuel consumption could get. About the same as a whale, I suppose! There would be a kind of conning tower with a deck for nice weather; but when things got rough, that would fold down to minimize drag and damage.

Would it work? I have the money to build one if it makes sense. Please comment on any pros or cons.

 

It makes sense. How do you feel about being submerged for a long time? I think that the psycological effect will prevent many people from accepting the concept.

 

Semi-submersible

Gonzo,
I would not expect to be submerged for long periods, just enough for the wave to pass over. And only then in heavy seas. A transparent skylight would ease clautrophobia.

I guess my view is that it is better to be submered occasionally than permanently! It is folklore that a large fisherman was tethered over the stern when a large wave came by, flooding the cockpit and sinking the vessel in seconds. I was sceptical but, if you calculate the volume of water taken aboard in the cockpit alone (say, 15 foot beam x 12 foot length x 3 feet deep) it would weigh around 33,000 pounds which would certaily be a problem. It seems to me that most large "cruisers" are fair-weather craft and simply could not survive a severe storm. Can you imaging doing a 360 rollover in a floating Winnebago! My aim is first to survive; everything else is optional.

 

For most, the joy of boating is not simply getting from A to B. It is the pleasure taken from simply being afloat - and all that it entails. If all you are after is a means of transport across the Atlantic, why not take a plane - it'll be faster, more comfortable, safer and MUCH MUCH cheaper!

by the way...

... a vessel will naturally trim to a position where the CG and CB are at the same point - that's one of the reasons for the ballast tanks that Mike referred to.

 

Will,
I already have a boat that I enjoy in fair weather and of course it is fun. The joy here, however, would be the sheer adventure of crossing from the US to Europe single handed - in reasonable safety. Besides, in good weather, I could sit topsides and enjoy the scenery like any other boat. The boat would only be about 70 percent submerged; more a wave piercer than a submarine.

I didn't understand your advice to make the CG and CB coincident. Surely the vessel would be neutrally stable and just as happy upside down as right side up. I was planning a very stable platform with the CG well below the CB - like a sailboat.

 

Sorry - you're right - I didn't explain that very well.
Let's work with longitudinal CB / CG - the same applies laterally as well though.
If the LCG is forward of the LCB then the vessel will trim down by the bow until the LCG and LCB are in vertical alignment. (As the bow is submerged, so the immersed volume forward is increased, which moves the LCB forward). The same applies if the LCG is aft of the LCB - the vessel will trim down by the stern until the two are aligned vertically.
In order to self-right, as you suggest the CG needs to be below the CB when the vessel is right-way-up

 

Guest,

That the vessel had high initial stability, low cg, low metacenter, significant lateral plane for motion dampening, a secure place for you to hang out in when the going got rough, impervious bouyancy chambers, and was self righting through 180 degrees would seem to be important to your concerns.

Whether your position in it was a few feet above or below the surface wouldn't seem to matter much. Either way your riding to the top of the wave you are worried about and if its breaking your going over on your head with all the associated excitement.

To avoid wave dynamics, you would have to submerge to what I would imagine would be an impractical depth for the low energy consumption, relatively low tech, craft you seem to be envisioning.

You might be better off to design for absolutely marginal bouyancy to minimize the vertical effects of wave motion.

Absolutely marginal bouyancy would seem to have some inherent risks of its own however.

Be advised I am not qualified to make any of these statements, they are only my thoughts as an interested layman.

Doug Carlson

 

I'm with you Doug. If I recall correctly wave energy extends to a depth equivalent to its height - so a 20 ft wave creates turbulence 20ft below the surface. In order to ensure that you didn't cop this you'd need a snorkel over 40ft long - and a 20 ft wave isn't exactly huge.

I would have thought that the cheapest way to do this would be to convert a motorised self-contained lifeboat - they're certainly seaworthy enough, and you'd have the added benefit of knowing that the design is well tested...

 

Will and Doug,
Thanks for some very good points. Doug's point that my main aim is to survive, and that I could make a safe vessel without it being a semi-submersible, is a good one. Will also suggeststed a lifeboat. However, I think my efficiency requirement would not be met with the lifeboat, but read on. Wills explanation of the longitudinal CG and CB is excellent. I had not thought of that; the less buoyancy the vessel has, the more critical that factor would become, especially if fuel loads shifted. Good stuff indeed

Now here is an insight that just arrived: All boats are partial submersibles because they all displace water. It is just the degree that is different. So, if we classify them according to their "percent submersible" factor (the percent of their volume submerged) a heavy monohull sailboat would be pretty high on that submersible scale (perhaps 50%?) whereas a sailing cat or a powerboat would be low (20%?). The higher mass of the monohull sailing boat is better for passagemaking because it slows the frequency of response to the waves. This prevents the pounding that the relatively lightweight powerboats suffer which makes for a much more comfortable ride in rough seas.

So, maybe what I am really looking for is a variation of a sailing boat hull (50% submersible) but with a narrow beam (since it has no sails it doesn't need a wide beam) and a heavy ballasted keel (torpedo type to minimize drag?). The bow would be sailboat pointy to pierce waves and the topsides would be clean to allow the big waves to pass over without too much drag. Oh yes, and strong hatches for when the bad stuff arrives. Wow! That sounds quite practical, and it could even look gorgeous. How about the length? About 30 or 40 feet seems about right to me with, perhaps, a 6 foot beam.
I am getting excited!

 

Birth of a Passagemaker

I think you have just reinvented the motorized passagemaker.

You are correct that the heavier displacement will provide a more comfortable ride and as long as you are willing to travel slowly, it is also very efficient.

The longer you make your length to beam ratio the more efficient it will be to power. The down side is the long length to beam tends to increase hull stress, so the ideal is probably in the 4:1 to 6:1 range.

Another problem is weight management. If you put the fuel down low it makes pretty good ballast, but as you consume fuel the empty tanks tend to destabilize the craft. This means that you need to have a great deal of ballast to stabilize it in the lightly loaded condition and then you need extra buoyancy to make her float fully loaded.

I have toyed with the idea of bladder tanks, like the racing boats use, except I would fill the bladder with sea water as I consume fuel. This allows you to maintain constant displacement and reduce your design range.

Good lock and keep us informed of your progress. If you are interested we could point you towards some similar designs to consider as a point of departure.

Regards;
Mike Schooley

 

I used to think heavy displacement "passagemakers" were better. After owning a 34" James Wharram catamaran with flexible beam mounts my opinion changed. It didn't pound and was more seakindly than any monohull I've ever been on. I sailed that boat in North Atlantic gales with 25' plus seas.

 

gettin to the hole(s) in the water idea(s). thats great. its worse than havin a hole in the hand though

 

When I was young and boating on lakes, I thought Catamarans and Trimarans were the only boats but then I started boating in the Ocean and spending nights onboard and I developed a sleep dependency.

My work requires me to spend a lot of time on high speed boats, generally with guns firing or missiles launching, so when I go on vacation I’m looking for the smooth, quite and safe ride of a full displacement or semi-displacement boat.

Regards;
Mike Schooley

 

Mike and Gonzo,
I replied but lost the message in the registration process!
Mike, My "passagemaker" is a lot smaller than the big tawlers that usually go by that name. Yes please, send me anything you can on vessels of a similar concept. I like your idea of using seawater to replace the spent fuel; it would be pretty easy to do with inflatable bags.
Gonzo, I know of the James Wharran cats. But even he tells of scary situations with them and recommends turning and running away from squalls. With my luck I would flip over.

I have re-read Dave Gerr's book on boat design and, on balance, think the monohull is more survivable. I would like to use a prismatic, seamless, rolled-steel hull (U-shaped section) with welded bulkheads and deck as the main structure, and use add-on molded or shaped bow and stern fairings for streamlining. The non-compond curve would be low cost and immensely strong. The bow and stern fairings would have a tubular frame support and filled with foam flotation. The interior bulheads would have waterproof doors. The weight and moment of intertia would be adjusted to give Dave's recommended roll and heaving frequency for a nice ride in heavy seas.

One basic question: If displacement hull speed is always proprtional to the square root of the length, why does a boat with a narrow beam go faster than one with a wide beam? Surely the wave-making drag is more on the wide boat? Or is it just that the wider boat needs more horsepower to reach its hull speed?